In recent years, there have been an increasing number of portable electronic devices equipped with rechargeable batteries, such as mobile phones and music players. At the same time, power transmission techniques have been developed to wirelessly supply power to such electronic devices.
The wireless power transmission techniques are of two types: an electromagnetic induction type using electromagnetic induction and a magnetic field resonance type using magnetic field resonance. The electromagnetic induction type involves power transmission in which a magnetic flux produced by a coil of a power transmitting device is received by a power receiving device. In the wireless power transmission of electromagnetic induction type, the power transmission efficiency is significantly influenced by a distance between the coil of the power transmitting device and the coil of the power receiving device. Specifically, the power transmission efficiency decreases significantly with increasing distance between the coil of the power transmitting device and the coil of the power receiving device.
On the other hand, the magnetic field resonance type involves use of a power transmitting device and a power receiving device each having a resonant circuit formed by a coil and a capacitor. Here, power is transmitted from the power transmitting device to the power receiving device by making resonance frequencies of the two resonant circuits equal. The wireless power transmission of magnetic field resonance type is advantageous in that even if there is some distance between the power transmitting device and the power receiving device, power may be transmitted with high efficiency. These related arts are disclosed, for example, in Japanese National Publication of International Patent Application No. 2009-504115, Japanese Laid-Open Patent Publication No. 2010-193701, Japanese Laid-Open Patent Publication No. 2010-98896, and Japanese Laid-Open Patent Publication No. 2005-110399.